Defining desmosomal plakophilin-3 interactions

Unveiling the Binding between the Armadillo-Repeat Domain of Plakophilin 1 and the Intrinsically Disordered Transcriptional Repressor RYBP

Plakophilin 1 (PKP1), a member of the p120ctn subfamily of the armadillo (ARM)-repeat-containing proteins, is an important structural component of cell-cell adhesion scaffolds although it can also be ubiquitously found in the cytoplasm and the nucleus. RYBP (RING 1A and YY1 binding protein) is a multifunctional intrinsically disordered protein (IDP) best described as a transcriptional regulator. Both proteins are involved in the development and metastasis of several types of tumors. We studied the binding of the armadillo domain of PKP1 (ARM-PKP1) with RYBP by using in cellulo methods, namely immunofluorescence (IF) and proximity ligation assay (PLA), and in vitro biophysical techniques, namely fluorescence, far-ultraviolet (far-UV) circular dichroism (CD), and isothermal titration calorimetry (ITC). We also characterized the binding of the two proteins by using in silico experiments. Our results showed that there was binding in tumor and non-tumoral cell lines. Binding in vitro between the two proteins was also monitored and found to occur with a dissociation constant in the low micromolar range (~10 μM). Finally, in silico experiments provided additional information on the possible structure of the binding complex, especially on the binding ARM-PKP1 hot-spot. Our findings suggest that RYBP might be a rescuer of the high expression of PKP1 in tumors, where it could decrease the epithelial-mesenchymal transition in some cancer cells.

The molecular architecture of the desmosomal outer dense plaque by integrative structural modeling

Desmosomes mediate cell-cell adhesion and are prevalent in tissues under mechanical stress. However, their detailed structural characterization is not available. Here, we characterized the molecular architecture of the desmosomal outer dense plaque (ODP) using Bayesian integrative structural modeling via the Integrative Modeling Platform. Starting principally from the structural interpretation of an electron cryo-tomogram, we integrated information from X-ray crystallography, an immuno-electron microscopy study, biochemical assays, in-silico predictions of transmembrane and disordered regions, homology modeling, and stereochemistry information. The integrative structure was validated by information from imaging, tomography, and biochemical studies that were not used in modeling. The ODP resembles a densely packed cylinder with a PKP layer and a PG layer; the desmosomal cadherins and PKP span these two layers. Our integrative approach allowed us to localize disordered regions, such as N-PKP and PG-C. We refined previous protein-protein interactions between desmosomal proteins and provided possible structural hypotheses for defective cell-cell adhesion in several diseases by mapping disease-related mutations on the structure. Finally, we point to features of the structure that could confer resilience to mechanical stress. Our model provides a basis for generating experimentally verifiable hypotheses on the structure and function of desmosomal proteins in normal and disease states.

Role and function of plakophilin 3 in cancer progression and skin disease

Plakophilin 3 (PKP3), a component of desmosome, is aberrantly expressed in many kinds of human diseases, especially in cancers. Through direct interaction, PKP3 binds with a series of desmosomal proteins, such as desmoglein, desmocollin, plakoglobin, and desmoplakin, to initiate desmosome aggregation, then promotes its stability. As PKP3 is mostly expressed in the skin, loss of PKP3 promotes the development of several skin diseases, such as paraneoplastic pemphigus, pemphigus vulgaris, and hypertrophic scar. Moreover, accumulated clinical data indicate that PKP3 dysregulates in diverse cancers, including breast, ovarian, colon, and lung cancers. Numerous lines of evidence have shown that PKP3 plays important roles in multiple cellular processes during cancer progression, including metastasis, invasion, tumor formation, autophagy, and proliferation. This review examines the diverse functions of PKP3 in regulating tumor formation and development in various types of cancers and summarizes its detailed mechanisms in the occurrence of skin diseases.

Plakophilin 2 regulates intestinal barrier function by modulating protein kinase C activity in vitro

Previous data provided evidence for a critical role of desmosomes to stabilize intestinal epithelial barrier (IEB) function. These studies suggest that desmosomes not only contribute to intercellular adhesion but also play a role as signaling hubs. The contribution of desmosomal plaque proteins plakophilins (PKP) in the intestinal epithelium remains unexplored. The intestinal expression of PKP2 and PKP3 was verified in human gut specimens, human intestinal organoids as well as in Caco2 cells whereas PKP1 was not detected. Knock-down of PKP2 using siRNA in Caco2 cells resulted in loss of intercellular adhesion and attenuated epithelial barrier. This was paralleled by changes of the whole desmosomal complex, including loss of desmoglein2, desmocollin2, plakoglobin and desmoplakin. In addition, tight junction proteins claudin1 and claudin4 were reduced following the loss of PKP2. Interestingly, siRNA-induced loss of PKP3 did not change intercellular adhesion and barrier function in Caco2 cells, while siRNA-induced loss of both PKP2 and PKP3 augmented the changes observed for reduced PKP2 alone. Moreover, loss of PKP2 and PKP2/3, but not PKP3, resulted in reduced activity levels of protein kinase C (PKC). Restoration of PKC activity using Phorbol 12-myristate 13-acetate (PMA) rescued loss of intestinal barrier function and attenuated the reduced expression patterns of claudin1 and claudin4. Immunostaining, proximity ligation assays and co-immunoprecipitation revealed a direct interaction between PKP2 and PKC. In summary, our in vitro data suggest that PKP2 plays a critical role for intestinal barrier function by providing a signaling hub for PKC-mediated expression of tight junction proteins claudin1 and claudin4.

The intercalated disc: a unique organelle for electromechanical synchrony in cardiomyocytes

The intercalated disc (ID) is a highly specialized structure that connects cardiomyocytes via mechanical and electrical junctions. Although described in some detail by light microscopy in the 19th century, it was in 1966 that electron microscopy images showed that the ID represented apposing cell borders and provided detailed insight into the complex ID nanostructure. Since then, much has been learned about the ID and its molecular composition, and it has become evident that a large number of proteins, not all of them involved in direct cell-to-cell coupling via mechanical or gap junctions, reside at the ID. Furthermore, an increasing number of functional interactions between ID components are emerging, leading to the concept that the ID is not the sum of isolated molecular silos but an interacting molecular complex, an "organelle" where components work in concert to bring about electrical and mechanical synchrony. The aim of the present review is to give a short historical account of the ID's discovery and an updated overview of its composition and organization, followed by a discussion of the physiological implications of the ID architecture and the local intermolecular interactions. The latter will focus on both the importance of normal conduction of cardiac action potentials as well as the impact on the pathophysiology of arrhythmias.

Plakophilin-3 Binds the Membrane and Filamentous Actin without Bundling F-Actin

Plakophilin-3 is a ubiquitously expressed protein found widely in epithelial cells and is a critical component of desmosomes. The plakophilin-3 carboxy-terminal domain harbors nine armadillo repeat motifs with largely unknown functions. Here, we report the 5 Å cryogenic electron microscopy (cryoEM) structure of the armadillo repeat motif domain of plakophilin-3, one of the smaller cryoEM structures reported to date. We find that this domain is a monomer or homodimer in solution. In addition, using an in vitro actin co-sedimentation assay, we show that the armadillo repeat domain of plakophilin-3 directly interacts with F-actin. This feature, through direct interactions with actin filaments, could be responsible for the observed association of extra-desmosomal plakophilin-3 with the actin cytoskeleton directly attached to the adherens junctions in A431 epithelial cells. Further, we demonstrate, through lipid binding analyses, that plakophilin-3 can effectively be recruited to the plasma membrane through phosphatidylinositol-4,5-bisphosphate-mediated interactions. Collectively, we report on novel properties of plakophilin-3, which may be conserved throughout the plakophilin protein family and may be behind the roles of these proteins in cell-cell adhesion.

The armadillo-repeat domain of Plakophilin 1 binds to human enzyme PADI4

Plakophilin 1 (PKP1), a member of the armadillo repeat family of proteins, is a key structural component of cell-cell adhesion scaffolds, although it can also be found in other cell locations, including the cytoplasm and the nucleus. PADI4 (peptidyl-arginine deiminase 4) is one of the human isoforms of a family of enzymes engaged in the conversion of arginine to citrulline, and is present in monocytes, macrophages, granulocytes, and in several types of cancer cells. It is the only family member observed both within the nucleus and the cytoplasm under ordinary conditions. We studied the binding of the armadillo domain of PKP1 (ARM-PKP1) with PADI4, by using several biophysical methods, namely fluorescence, far-ultraviolet (far-UV) circular dichroism (CD), isothermal titration calorimetry (ITC), and molecular simulations; furthermore, binding was also tested by Western-blot (WB) analyses. Our results show that there was binding between the two proteins, with a dissociation constant in the low micromolar range (∼ 1 μM). Molecular modelling provided additional information on the possible structure of the binding complex, and especially on the binding hot-spot predicted for PADI4. This is the first time that the interaction between these two proteins has been described and studied. Our findings could be of importance to understand the development of tumors, where PKP1 and PADI4 are involved. Moreover, our findings pave the way to describe the formation of neutrophil extracellular traps (NETs), whose construction is modulated by PADI4, and which mediate the proteolysis of cell-cell junctions where PKP1 intervenes.

Proteomic profiling of sweat in patients with cystic fibrosis provides new insights into epidermal homoeostasis

Background

A high proportion of patients with Cystic Fibrosis (CF) also present the rare skin disease aquagenic palmoplantar keratoderma. A possible link between this condition and absence of a functional CF Transmembrane conductance Regulator protein in the sweat acinus and collecting duct remains unknown.

Methods

In-depth characterization of sweat proteome profiles was performed in 25 CF patients compared to 12 healthy controls. A 20 μL sweat sample was collected after pilocarpine iontophoresis and liquid chromatography tandem mass spectrometry (LC-MS/MS) proteomic analysis was performed.

Results

Sweat proteome profile of CF patients was significantly different from that of healthy subjects with 57 differentially expressed proteins. Cystic Fibrosis sweat proteome was characterized by an increase in 25 proteins including proteases (Kallikrein 7 and 13, Phospholipase B domain containing 1, Cathepsin A L2 and B, Lysosomal Pro-X carboxypeptidase); proinflammatory proteins (Annexin A2, Chitinase-3-like protein 1); cytochrome c and transglutaminases. Thirty-two proteins were downregulated in CF sweat including proteases (Elastase 2), antioxidative protein FAM129 B; membrane-bound transporter SLC6A14 and regulator protein Sodium-hydrogen antiporter 3 regulator 1.

Conclusion

This study is the first to report in-depth characterization of endogenous peptides in CF sweat and could help understand the complex physiology of the sweat gland. The proteome profile highlights the unbalanced proteolytic and proinflammatory activity of sweat in CF. These results also suggest a defect in pathways involved in skin barrier integrity in CF patients. Sweat proteome profile could prove to be a useful tool in the context of personalized medicine in CF.

Clostridium novyi’s Alpha-Toxin Changes Proteome and Phosphoproteome of HEp-2 Cells

C. novyi type A produces the alpha-toxin (TcnA) that belongs to the large clostridial glucosylating toxins (LCGTs) and is able to modify small GTPases by N-acetylglucosamination on conserved threonine residues. In contrast, other LCGTs including Clostridioides difficile toxin A and toxin B (TcdA; TcdB) modify small GTPases by mono-o-glucosylation. Both modifications inactivate the GTPases and cause strong effects on GTPase-dependent signal transduction pathways and the consequent reorganization of the actin cytoskeleton leading to cell rounding and finally cell death. However, the effect of TcnA on target cells is largely unexplored. Therefore, we performed a comprehensive screening approach of TcnA treated HEp-2 cells and analyzed their proteome and their phosphoproteome using LC-MS-based methods. With this data-dependent acquisition (DDA) approach, 5086 proteins and 9427 phosphosites could be identified and quantified. Of these, 35 proteins were found to be significantly altered after toxin treatment, and 1832 phosphosites were responsive to TcnA treatment. By analyzing the TcnA-induced proteomic effects of HEp-2 cells, 23 common signaling pathways were identified to be altered, including Actin Cytoskeleton Signaling, Epithelial Adherens Junction Signaling, and Signaling by Rho Family GTPases. All these pathways are also regulated after application of TcdA or TcdB of C. difficile. After TcnA treatment the regulation on phosphorylation level was much stronger compared to the proteome level, in terms of both strength of regulation and the number of regulated phosphosites. Interestingly, various signaling pathways such as Signaling by Rho Family GTPases or Integrin Signaling were activated on proteome level while being inhibited on phosphorylation level or vice versa as observed for the Role of BRCA1 in DNA Damage Response. ZIP kinase, as well as Calmodulin-dependent protein kinases IV & II, were observed as activated while Aurora-A kinase and CDK kinases tended to be inhibited in cells treated with TcnA based on their substrate regulation pattern.

Case Report: Bi-allelic missense variant in the desmocollin 3 gene causes hypotrichosis and recurrent skin vesicles

Background: Hypotrichosis with Recurrent Skin Vesicles (HYPTSV) is an extremely rare condition, having autosomal recessive inheritance. Here in we report a 4-years- old Saudi boy who presented with a history of recurrent skin blisters that are localized to the extremities and hypotrichosis since birth.

Methods: The present study describes a consanguineous Saudi family segregating HYPTSV in an autosomal recessive fashion. A single proband (II-1) exhibited features such as diffused non-scarring alopecia on the scalp, intraepidermal blister, post-inflammatory hyperpigmented macules, and follicular hyperkeratosis. DNA of the index was subjected to whole-genome sequencing (WGS). Furthermore, 3D protein modeling was performed for the mutated and normal protein.

Results: WGS revealed a novel bi-allelic missense variant (c.154G>C; p. Val52Leu) in the DSC3 gene, which segregated perfectly using Sanger sequencing. In addition, 3D protein modeling revealed a substantial change in the mutated DSC3 protein as compared to the normal DSC3 protein.

Conclusion: This is the 3rd novel variant reported in the DSC3 gene associated with the HYPTSV phenotype. This report further strengthens the evidence that bi-allelic variants in the DSC3 cause severe HYPTSV in humans.

Associations of Heavy Metals with Activities of Daily Living Disability: An Epigenome-Wide View of DNA Methylation and Mediation Analysis

BACKGROUND

Exposure to heavy metals has been reported to be associated with multiple diseases. However, direct associations and potential mechanisms of heavy metals with physical disability remain unclear.

OBJECTIVES

We aimed to quantify associations of heavy metals with physical disability and further explore the potential mechanisms of DNA methylation on the genome scale.

METHODS

A cross-sectional study of 4,391 older adults was conducted and activities of daily living (ADL) disability were identified using a 14-item scale questionnaire including basic and instrumental activities to assess the presence of disability (yes or no) rated on a scale of dependence. Odds ratios (ORs) and 95% confidence intervals (CI) were estimated to quantify associations between heavy metals and ADL disability prevalence using multivariate logistic regression and Bayesian kernel machine regression (BKMR) models. Whole blood-derived DNA methylation was measured using the HumanMethylationEPIC BeadChip array. An ADL disability-related epigenome-wide DNA methylation association study (EWAS) was performed among 212 sex-matched ADL disability cases and controls, and mediation analysis was further applied to explore potential mediators of DNA methylation.

RESULTS

Each 1-standard deviation (SD) higher difference in log10-transformed manganese, copper, arsenic, and cadmium level was significantly associated with a 14% (95% CI: 1.05, 1.24), 16% (95% CI:1.07, 1.26), 22% (95% CI:1.13, 1.33), and 15% (95% CI:1.06, 1.26) higher odds of ADL disability, which remained significant in the multiple-metal and BKMR models. A total of 85 differential DNA methylation sites were identified to be associated with ADL disability prevalence, among which methylation level at cg220000984 and cg23012519 (annotated to IRGM and PKP3) mediated 31.0% and 31.2% of manganese-associated ADL disability prevalence, cg06723863 (annotated to ESRP2) mediated 32.4% of copper-associated ADL disability prevalence, cg24433124 (nearest to IER3) mediated 15.8% of arsenic-associated ADL disability prevalence, and cg07905190 and cg17485717 (annotated to FREM1 and TCP11L1) mediated 21.5% and 30.5% of cadmium-associated ADL disability prevalence (all p<0.05).

DISCUSSION

Our findings suggested that heavy metals contributed to higher prevalence of ADL disability and that locus-specific DNA methylation are partial mediators, providing potential biomarkers for further cellular mechanism studies. https://doi.org/10.1289/EHP10602.

YAP Inhibition by Verteporfin Causes Downregulation of Desmosomal Genes and Proteins Leading to the Disintegration of Intercellular Junctions

The Hippo-YAP pathway serves as a central signalling hub in epithelial tissue generation and homeostasis. Yes-associated protein (YAP) is an essential downstream transcription cofactor of this pathway, with its activity being negatively regulated by Hippo kinase-mediated phosphorylation, leading to its cytoplasmic translocation or degradation. Our recent study showed phospho-YAP complexes with Desmoglein-3 (Dsg3), the desmosomal cadherin known to be required for junction assembly and cell–cell adhesion. In this study, we show that YAP inhibition by Verteporfin (VP) caused a significant downregulation of desmosomal genes and a remarkable reduction in desmosomal proteins, including the Dsg3/phospho-YAP complex, resulting in attenuation of cell cohesion. We also found the desmosomal genes, along with E-cadherin, were the YAP-TEAD transcriptional targets and Dsg3 regulated key Hippo components, including WWTR1/TAZ, LATS2 and the key desmosomal molecules. Furthermore, Dsg3 and phospho-YAP exhibited coordinated regulation in response to varied cell densities and culture durations. Overexpression of Dsg3 could compensate for VP mediated loss of adhesion components and proper architecture of cell junctions. Thus, our findings suggest that Dsg3 plays a crucial role in the Hippo network and regulates junction configuration via complexing with phospho-YAP.

Plakophilin3 loss leads to increased adenoma formation and rectal prolapse in APCmin mice

Plakophilin3 (PKP3) loss leads to tumor progression and metastasis of colon cancer cells. The goal of this report was to determine if PKP3 loss led to increased disease progression in mice. We generated a colonocyte-specific knockout of PKP3 in APC^min mice, which led to increased adenoma formation, the formation of rectal prolapse, and a significant decrease in survival. The observed increase in rectal prolapse formation and decrease in survival correlated with an increase in the expression of Lipocalin2 (LCN2). Increased disease progression was observed even upon treatment with 5-fluorouracil (5FU). These results suggest that an increase in LCN2 expression might lead to therapy resistance and that LCN2 might serve as a potential therapeutic target in colorectal cancer.

Disruption of desmosome function leads to increased centrosome clustering in 14-3-3γ-knockout cells with supernumerary centrosomes

14-3-3 proteins are conserved, dimeric, acidic proteins that regulate multiple cellular pathways. Loss of either 14-3-3ε or 14-3-3γ leads to centrosome amplification. However, we find that while the knockout of 14-3-3ε leads to multipolar mitoses, the knockout of 14-3-3γ results in centrosome clustering and pseudo-bipolar mitoses. 14-3-3γ knockouts demonstrate compromised desmosome function and a decrease in keratin levels, leading to decreased cell stiffness and an increase in centrosome clustering. Restoration of desmosome function increased multipolar mitoses, whereas knockdown of either plakoglobin or keratin 5 led to decreased cell stiffness and increased pseudo-bipolar mitoses. These results suggest that the ability of the desmosome to anchor keratin filaments maintains cell stiffness, thus inhibiting centrosome clustering, and that phenotypes observed upon 14-3-3 loss reflect the dysregulation of multiple pathways.

Disease-related blood-based differential methylation in cystic fibrosis and its representation in lung cancer revealed a regulatory locus in PKP3 in lung epithelial cells

Cystic fibrosis (CF) is a monogenic disease, characterized by massive chronic lung inflammation. The observed variability in clinical phenotypes in monozygotic CF twins is likely associated with the extent of inflammation. This study sought to investigate inflammation-related aberrant DNA methylation in CF twins and to determine to what extent acquired methylation changes may be associated with lung cancer.

Blood-based genome-wide DNA methylation analysis was performed to compare the DNA methylomes of monozygotic twins, from the European CF Twin and Sibling Study with various degrees of disease severity. Putatively inflammation-related and differentially methylated positions were selected from a large lung cancer case-control study and investigated in blood by targeted bisulphite next-generation-sequencing. An inflammation-related locus located in the Plakophilin-3 (PKP3) gene was functionally analysed regarding promoter and enhancer activity in presence and absence of methylation using luciferase reporter assays.

We confirmed in a unique cohort that monozygotic twins, even if clinically discordant, have only minor differences in global DNA methylation patterns and blood cell composition. Further, we determined the most differentially methylated positions, a high proportion of which are blood cell-type-specific, whereas others may be acquired and thus have potential relevance in the context of inflammation as lung cancer risk factors. We identified a sequence in the gene body of PKP3 which is hypermethylated in blood from CF twins with severe phenotype and highly variably methylated in lung cancer patients and controls, independent of known clinical parameters, and showed that this region exhibits methylation-dependent promoter activity in lung epithelial cells.

Autoantibody-Specific Signalling in Pemphigus

Pemphigus is a severe autoimmune disease impairing barrier functions of epidermis and mucosa. Autoantibodies primarily target the desmosomal adhesion molecules desmoglein (Dsg) 1 and Dsg 3 and induce loss of desmosomal adhesion. Strikingly, autoantibody profiles in pemphigus correlate with clinical phenotypes. Mucosal-dominant pemphigus vulgaris (PV) is characterised by autoantibodies (PV-IgG) against Dsg3 whereas epidermal blistering in PV and pemphigus foliaceus (PF) is associated with autoantibodies against Dsg1. Therapy in pemphigus is evolving towards specific suppression of autoantibody formation and autoantibody depletion. Nevertheless, during the acute phase and relapses of the disease additional treatment options to stabilise desmosomes and thereby rescue keratinocyte adhesion would be beneficial. Therefore, the mechanisms by which autoantibodies interfere with adhesion of desmosomes need to be characterised in detail. Besides direct inhibition of Dsg adhesion, autoantibodies engage signalling pathways interfering with different steps of desmosome turn-over. With this respect, recent data indicate that autoantibodies induce separate signalling responses in keratinocytes via specific signalling complexes organised by Dsg1 and Dsg3 which transfer the signal of autoantibody binding into the cell. This hypothesis may also explain the different clinical pemphigus phenotypes.

Plakophilin 3 and Par3 facilitate desmosomes' association with the apical junctional complex

Desmosomes (DSMs), together with adherens junctions (AJs) and tight junctions (TJs), constitute the apical cell junctional complex (AJC). While the importance of the apical and basolateral polarity machinery in the organization of AJs and TJs is well established, how DSMs are positioned within the AJC is not understood. Here we use highly polarized DLD1 cells as a model to address how DSMs integrate into the AJC. We found that knockout (KO) of the desmosomal ARM protein Pkp3, but not other major DSM proteins, uncouples DSMs from the AJC without blocking DSM assembly. DLD1 cells also exhibit a prominent extraDSM pool of Pkp3, concentrated in tricellular (tC) contacts. Probing distinct apicobasal polarity pathways revealed that neither the DSM’s association with AJC nor the extraDSM pool of Pkp3 are abolished in cells with defects in Scrib module proteins responsible for basolateral membrane development. However, a loss of the apical polarity protein, Par3, completely eliminates the extraDSM pool of Pkp3 and disrupts AJC localization of desmosomes, dispersing these junctions along the entire length of cell–cell contacts. Our data are consistent with a model whereby Par3 facilitates DSM assembly within the AJC, controlling the availability of an assembly competent pool of Pkp3 stored in tC contacts.

The armadillo-repeat domain of plakophilin 1 binds the C-terminal sterile alpha motif (SAM) of p73

BACKGROUND

Plakophilin 1 (PKP1) is a component of desmosomes, which are key structural components for cell-cell adhesion, and can also be found in other cell locations. The p53, p63 and p73 proteins belong to the p53 family of transcription factors, playing crucial roles in tumour suppression. The α-splice variant of p73 (p73α) has at its C terminus a sterile alpha motif (SAM); such domain, SAMp73, is involved in the interaction with other macromolecules.

METHODS

We studied the binding of SAMp73 with the armadillo domain of PKP1 (ARM-PKP1) in the absence and the presence of 100 mM NaCl, by using several biophysical techniques, namely fluorescence, far-ultraviolet circular dichroism (CD), nuclear magnetic resonance (NMR), isothermal titration calorimetry (ITC), and molecular docking and simulations.

RESULTS

Association was observed between the two proteins, with a dissociation constant of ~5 μM measured by ITC and fluorescence in the absence of NaCl. The binding region of SAMp73 involved residues of the so-called "middle-loop-end-helix" binding region (i.e., comprising the third helix, together with the C terminus of the second one, and the N-cap of the fourth), as shown by ¹⁵N, ¹H- HSQC-NMR spectra. Molecular modelling provided additional information on the possible structure of the binding complex.

CONCLUSIONS

This newly-observed interaction could have potential therapeutic relevance in the tumour pathways where PKP1 is involved, and under conditions when there is a possible inactivation of p53.

GENERAL SIGNIFICANCE

The discovery of the binding between SAMp73 and ARM-PKP1 suggests a functional role for their interaction, including the possibility that SAMp73 could assist PKP1 in signalling pathways.

Intrinsically disordered protein NUPR1 binds to the armadillo-repeat domain of Plakophilin 1

Plakophilin 1 (PKP1), a member of the armadillo repeat family of proteins, is a scaffold component of desmosomes, which are key structural components for cell-cell adhesion. However, PKP1 can be also found in the nucleus of several cells. NUPR1 is an intrinsically disordered protein (IDP) that localizes throughout the whole cell, and intervenes in the development and progression of several cancers. In this work, we studied the binding between PKP1 and NUPR1 by using several in vitro biophysical techniques and in cellulo approaches. The interaction occurred with an affinity in the low micromolar range (~10 μM), and involved the participation of at least one of the tryptophan residues of PKP1 (as shown by fluorescence and molecular docking). The binding region of NUPR1, mapped by NMR and molecular modelling, was a polypeptide patch at the 30s region of its sequence. The association between PKP1 and NUPR1 also occurred in cellulo and was localized in the nucleus, as tested by protein ligation assays (PLAs). We hypothesize that NUPR1 plays an active role in carcinogenesis modulating the function of PKP1.

Identification of PKP 2/3 as potential biomarkers of ovarian cancer based on bioinformatics and experiments

Background

Plakophilins (PKPs) are widely involved in gene transcription, translation, and signal transduction, playing a crucial role in tumorigenesis and progression. However, the function and potential mechanism of PKP1/2/3 in ovarian cancer (OC) remains unclear. It’s of great value to explore the expression and prognostic values of PKP1/2/3 and their potential mechanisms, immune infiltration in OC.

Methods

The expression levels, prognostic values and genetic variations of PKP1/2/3 in OC were explored by various bioinformatics tools and databases, and PKP2/3 were selected for further analyzing their regulation network and immune infiltration. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes pathways (KEGG) enrichment were also conducted. Finally, the expression and prognosis of PKP2 were validated by immunohistochemistry.

Results

The expression level and prognosis of PKP1 showed little significance in ovarian cancer, and the expression of PKP2/3 mRNA and protein were upregulated in OC, showing significant correlations with poor prognosis of OC. Functional enrichment analysis showed that PKP2/3 and their correlated genes were significantly enriched in adaptive immune response, cytokine receptor activity, organization of cell–cell junction and extracellular matrix; KEGG analysis showed that PKP2/3 and their significantly correlated genes were involved in signaling pathways including cytokine-mediated signaling pathway, receptor signaling pathway and pathways in cancer. Moreover, PKP2/3 were correlated with lymphocytes and immunomodulators. We confirmed that high expression of PKP2 was significantly associated with advanced stage, poor differentiation and poor prognosis of OC patients.

Conclusion

Members of plakophilins family showed various degrees of abnormal expressions and prognostic values in ovarian cancer. PKP2/3 played crucial roles in tumorigenesis, aggressiveness, malignant biological behavior and immune infiltration of OC, and can be regarded as potential biomarker for early diagnosis and prognosis evaluation in OC.

Mutations in genes encoding desmosomal proteins: spectrum of cutaneous and extracutaneous abnormalities

The desmosome is a type of intercellular junction found in epithelial cells, cardiomyocytes and other specialized cell types. Composed of a network of transmembranous cadherins and intracellular armadillo, plakin and other proteins, desmosomes contribute to cell-cell adhesion, signalling, development and differentiation. Mutations in genes encoding desmosomal proteins result in a spectrum of erosive skin and mucosal phenotypes that also may affect hair or heart. This review summarizes the molecular pathology and phenotypes associated with desmosomal dysfunction with a focus on inherited disorders that involve the skin/hair, as well as associated extracutaneous pathologies. We reviewed the relevant literature to collate studies of pathogenic human mutations in desmosomes that have been reported over the last 25 years. Mutations in 12 different desmosome genes have been documented, with mutations in nine genes affecting the skin/mucous membranes (DSG1, DSG3, DSC2, DSC3, JUP, PKP1, DSP, CDSN, PERP) and eight resulting in hair abnormalities (DSG4, DSC2, DSC3, JUP, PKP1, DSP, CDSN, PERP). Mutations in three genes can result in cardiocutaneous syndromes (DSC2, JUP, DSP), although mutations have been described in five genes in inherited heart disorders that may lack any dermatological manifestations (DSG2, DSC2, JUP, PKP2, DSP). Understanding the diverse nature of these clinical phenotypes, as well as the desmosome gene mutation(s), has clinical value in managing and counselling patients, as well as demonstrating the biological role and activity of specific components of desmosomes in skin and other tissues.

The isolated armadillo-repeat domain of Plakophilin 1 is a monomer in solution with a low conformational stability

Plakophilin 1 (PKP1) is a member of the armadillo repeat family of proteins. It serves as a scaffold component of desmosomes, which are key structural components for cell-cell adhesion. We have embarked on the biophysical and conformational characterization of the ARM domain of PKP1 (ARM-PKP1) in solution by using several spectroscopic (namely, fluorescence and circular dichroism (CD)) and biophysical techniques (namely, analytical ultracentrifugation (AUC), dynamic light scattering (DLS) and differential scanning calorimetry (DSC)). ARM-PKP1 was a monomer in solution at physiological pH, with a low conformational stability, as concluded from DSC experiments and thermal denaturations followed by fluorescence and CD. The presence or absence of disulphide bridges did not affect its low stability. The protein unfolded through an intermediate which has lost native-like secondary structure. ARM-PKP1 acquired a native-like structure in a narrow pH range (between pH 6.0 and 8.0), indicating that its adherent properties might only work in a very narrow pH range.

The Barrier Molecules Junction Plakoglobin, Filaggrin, and Dystonin Play Roles in Melanoma Growth and Angiogenesis

OBJECTIVE

To understand role of barrier molecules in melanomas.

BACKGROUND

We have reported poor patient survival and low immune infiltration of melanomas that overexpress a set of genes that include filaggrin (FLG), dystonin (DST), junction plakoglobin (JUP), and plakophilin-3 (PKP3), and are involved in cell-cell adhesions. We hypothesized that these associations are causal, either by interfering with immune cell infiltration or by enhancing melanoma cell growth.

METHODS

FLG and DST were knocked out by CRISPR/Cas9 in human DM93 and murine B16-F1 melanoma cells. PKP3 and JUP were overexpressed in murine B16-AAD and human VMM39 melanoma cells by lentiviral transduction. These cell lines were evaluated in vitro for cell proliferation and in vivo for tumor burden, immune composition, cytokine expression, and vascularity.

RESULTS

Immune infiltrates were not altered by these genes. FLG/DST knockout reduced proliferation of human DM93 melanoma in vitro, and decreased B16-F1 tumor burden in vivo. Overexpression of JUP, but not PKP3, in B16-AAD significantly increased tumor burden, increased VEGF-A, reduced IL-33, and enhanced vascularity.

CONCLUSIONS

FLG and DST support melanoma cell growth in vitro and in vivo. Growth effects of JUP were only evident in vivo, and may be mediated, in part, by enhancing angiogenesis. In addition, growth-promoting effects of FLG and DST in vitro suggest that these genes may also support melanoma cell proliferation through angiogenesis-independent pathways. These findings identify FLG, DST, and JUP as novel therapeutic targets whose down-regulation may provide clinical benefit to patients with melanoma.

Exploring disease-specific methylated CpGs in human male genital abnormalities by using methylated-site display-amplified fragment length polymorphism (MSD-AFLP)

The incidence of male reproductive system disorders, especially hypospadias, has been increasing in developed countries since the latter half of the 20th century. Endocrine-disrupting chemicals from the environment are considered to be involved in hypospadias onset through epigenetic alterations. This pilot study aimed to explore disease-specific methylated CpGs in human patient samples using the methylated-site display-amplified fragment length polymorphism (MSD-AFLP) technique developed by our research group [1]. We compared clinical samples from hypospadias and phimosis patients. Foreskin and blood samples were collected from one- to two-year-old patients with hypospadias (N = 3) and phimosis (N = 3) during surgical treatment. MSD-AFLP analysis showed significantly decreased CpG-methylation levels of genes such as MYH11 and increased CpG-methylation levels of genes such as PLA2G15 in hypospadias patients. Hierarchical clustering analysis showed that genes with significantly altered CpG levels were more markedly altered in DNA from blood than from foreskin. Because of the small number of samples, further investigation is necessary to elucidate the association between variations in CpG levels in foreskin and blood DNA and male genital abnormalities. However, our MSD-AFLP method appears to be a useful tool for exploring disease-specific methylated-CpGs in human epidemiological studies.

Evidence for the Desmosomal Cadherin Desmoglein-3 in Regulating YAP and Phospho-YAP in Keratinocyte Responses to Mechanical Forces

Desmoglein 3 (Dsg3) plays a crucial role in cell-cell adhesion and tissue integrity. Increasing evidence suggests that Dsg3 acts as a regulator of cellular mechanotransduction, but little is known about its direct role in mechanical force transmission. The present study investigated the impact of cyclic strain and substrate stiffness on Dsg3 expression and its role in mechanotransduction in keratinocytes. A direct comparison was made with E-cadherin, a well-characterized mechanosensor. Exposure of oral and skin keratinocytes to equiaxial cyclic strain promoted changes in the expression and localization of junction assembly proteins. The knockdown of Dsg3 by siRNA blocked strain-induced junctional remodeling of E-cadherin and Myosin IIa. Importantly, the study demonstrated that Dsg3 regulates the expression and localization of yes-associated protein (YAP), a mechanosensory, and an effector of the Hippo pathway. Furthermore, we showed that Dsg3 formed a complex with phospho-YAP and sequestered it to the plasma membrane, while Dsg3 depletion had an impact on both YAP and phospho-YAP in their response to mechanical forces, increasing the sensitivity of keratinocytes to the strain or substrate rigidity-induced nuclear relocation of YAP and phospho-YAP. Plakophilin 1 (PKP1) seemed to be crucial in recruiting the complex containing Dsg3/phospho-YAP to the cell surface since its silencing affected Dsg3 junctional assembly with concomitant loss of phospho-YAP at the cell periphery. Finally, we demonstrated that this Dsg3/YAP pathway has an influence on the expression of YAP1 target genes and cell proliferation. Together, these findings provide evidence of a novel role for Dsg3 in keratinocyte mechanotransduction.

Plakophilin 1 but not plakophilin 3 regulates desmoglein clustering

Plakophilins (Pkp) are desmosomal plaque proteins crucial for desmosomal adhesion and participate in the regulation of desmosomal turnover and signaling. However, direct evidence that Pkps regulate clustering and molecular binding properties of desmosomal cadherins is missing. Here, keratinocytes lacking either Pkp1 or 3 in comparison to wild type (wt) keratinocytes were characterized with regard to their desmoglein (Dsg) 1- and 3-binding properties and their capability to induce Dsg3 clustering. As revealed by atomic force microscopy (AFM), both Pkp-deficient keratinocyte cell lines showed reduced membrane availability and binding frequency of Dsg1 and 3 at cell borders. Extracellular crosslinking and AFM cluster mapping demonstrated that Pkp1 but not Pkp3 is required for Dsg3 clustering. Accordingly, Dsg3 overexpression reconstituted cluster formation in Pkp3- but not Pkp1-deficient keratinocytes as shown by AFM and STED experiments. Taken together, these data demonstrate that both Pkp1 and 3 regulate Dsg membrane availability, whereas Pkp1 but not Pkp3 is required for Dsg3 clustering.

Armc8 is an evolutionarily conserved armadillo protein involved in cell-cell adhesion complexes through multiple molecular interactions

Armadillo-repeat-containing protein 8 (Armc8) belongs to the family of armadillo-repeat containing proteins, which have been found to be involved in diverse cellular functions including cell-cell contacts and intracellular signaling. By comparative analyses of armadillo repeat protein structures and genomes from various premetazoan and metazoan species, we identified orthologs of human Armc8 and analyzed in detail the evolutionary relationship of Armc8 genes and their encoded proteins. Armc8 is a highly ancestral armadillo protein although not present in yeast. Consequently, Armc8 is not the human ortholog of yeast Gid5/Vid28.Further, we performed a candidate approach to characterize new protein interactors of Armc8. Interactions between Armc8 and specific δ-catenins (plakophilins-1, -2, -3 and p0071) were observed by the yeast two-hybrid approach and confirmed by co-immunoprecipitation and co-localization. We also showed that Armc8 interacts specifically with αE-catenin but neither with αN-catenin nor with αT-catenin. Degradation of αE-catenin has been reported to be important in cancer and to be regulated by Armc8. A similar process may occur with respect to plakophilins in desmosomes. Deregulation of desmosomal proteins has been considered to contribute to tumorigenesis.

Incidentally identified genetic variants in arrhythmogenic right ventricular cardiomyopathy-associated genes among children undergoing exome sequencing reflect healthy population variation

BACKGROUND

With expanding use of clinical whole exome sequencing (WES), genetic variants of uncertain significance are increasingly identified. As pathologic mutations in genes associated with arrhythmogenic right ventricular cardiomyopathy (ARVC) carry a risk of sudden death, determining the diagnostic relevance of incidentally identified variants associated with these genes is critical.

METHODS

WES variants from a large, predominantly pediatric cohort (N = 7,066 probands) were obtained for nine ARVC-associated genes (Baylor Miraca). For comparison, a control cohort was derived from the gnomAD database and an ARVC case cohort (N = 1,379 probands) was established from ARVC cases in the literature. Topologic mapping was performed and signal-to-noise analysis was conducted normalizing WES, or case variants, against control variant frequencies. Retrospective chart review was performed of WES cases evaluated clinically (Texas Children's Hospital).

RESULTS

Incidentally identified variants occurred in 14% of WES referrals and localized to genes which were rare among ARVC cases yet similar to controls. Amino acid-level signal-to-noise analysis of cases demonstrated "pathologic hotspots" localizing to critical domains of PKP2 and DSG2 while WES variants did not. PKP2 ARM7 and ARM8 domains and DSG2 N-terminal cadherin-repeat domains demonstrated high pathogenicity while normalized WES variant frequency was low. Review of clinical data available on WES referrals demonstrated none with evidence of ARVC among variant-positive individuals.

CONCLUSIONS

Incidentally identified variants are common among pediatric WES testing with gene frequencies similar to "background" variants. Incidentally identified variants are unlikely to be pathologic.

Increased expression of plakophilin 3 is associated with poor prognosis in ovarian cancer

Considering the essential role of plakophilin 3 (PKP3) in the maintenance cell-cell adhesion, dysregulation of PKP3 is involved in human diseases. This study aimed to explore the clinical significance of PKP3 in ovarian cancer. Immunohistochemistry was performed to examine the PKP3 expression in 157 cancer specimens from primary ovarian cancer patients. PKP3 was expressed in both the cytoplasm and nucleus. Eighty-one (51.6%) out of 157 ovarian cancer tissues showed PKP3 expression, while absent expression was observed in normal ovarian tissues. High PKP3 expression was associated with lymph node metastasis (LNM, P = .004) and advanced International Federation of Gynecology and Obstetrics (FIGO) stage (P = .013). Patients with high PKP3 expression had shorter overall survival (OS) than those with low PKP3 expression (60.2 months vs 74.2 months, P = .021). However, no association between PKP3 expression and progression-free survival (PFS) was observed (P = .790). Cox regression analysis indicated that PKP3 expression was an independently predictive factor for the OS of patient with ovarian cancer (adjusted HR = 1.601, 95%CI: 1.014-2.528, P = .043), especially those with FIGO stages III and IV disease (adjusted HR = 1.607, 95%CI: 1.006-2.567, P = .047). The gene expression profiling interactive analysis (GEPIA) databases also showed that PKP3 was upregulated in ovarian cancer (P < .001) and patients with high PKP3 expression had shorter OS (P = .004). In conclusion, our findings suggest that PKP3 is upregulated in ovarian cancer and is likely involved in the progression of ovarian cancer. PKP3 might therefore serve as a prognostic biomarker for patients with ovarian cancer.

Non-Desmoglein Antibodies in Patients With Pemphigus Vulgaris

Pemphigus vulgaris (PV) is a potentially life-threatening mucocutaneous autoimmune blistering disease. Patients develop non-healing erosions and blisters due to cell–cell detachment of keratinocytes (acantholysis), with subsequent suprabasal intraepidermal splitting. Identified almost 30 years ago, desmoglein-3 (Dsg3), a Ca²⁺-dependent cell adhesion molecule belonging to the cadherin family, has been considered the “primary” autoantigen in PV. Proteomic studies have identified numerous autoantibodies in patients with PV that have known roles in the physiology and cell adhesion of keratinocytes. Antibodies to these autoantibodies include desmocollins 1 and 3, several muscarinic and nicotinic acetylcholine receptor subtypes, mitochondrial proteins, human leukocyte antigen molecules, thyroid peroxidase, and hSPCA1—the Ca²⁺/Mn²⁺-ATPase encoded by ATP2C1, which is mutated in Hailey–Hailey disease. Several studies have identified direct pathogenic roles of these proteins, or synergistic roles when combined with Dsg3. We review the role of these direct and indirect mechanisms of non-desmoglein autoantibodies in the pathogenesis of PV.

Plakophilin3 loss leads to an increase in lipocalin2 expression, which is required for tumour formation

An increase in tumour formation and metastasis are observed upon plakophilin3 (PKP3) loss. To identify pathways downstream of PKP3 loss that are required for increased tumour formation, a gene expression analysis was performed, which demonstrated that the expression of lipocalin2 (LCN2) was elevated upon PKP3 loss and this is consistent with expression data from human tumour samples suggesting that PKP3 loss correlates with an increase in LCN2 expression. PKP3 loss leads to an increase in invasion, tumour formation and metastasis and these phenotypes were dependent on the increase in LCN2 expression. The increased LCN2 expression was due to an increase in the activation of p38 MAPK in the HCT116 derived PKP3 knockdown clones as LCN2 expression decreased upon inhibition of p38 MAPK. The phosphorylated active form of p38 MAPK is translocated to the nucleus upon PKP3 loss and is dependent on complex formation between p38 MAPK and PKP3. WT PKP3 inhibits LCN2 reporter activity in PKP3 knockdown cells but a PKP3 mutant that fails to form a complex with p38 MAPK cannot suppress LCN2 promoter activity. Further, LCN2 expression is decreased upon loss of p38β, but not p38α, in the PKP3 knockdown cells. These results suggest that PKP3 loss leads to an increase in the nuclear translocation of p38 MAPK and p38β MAPK is required for the increase in LCN2 expression.

Beyond cell-cell adhesion: Plakoglobin and the regulation of tumorigenesis and metastasis

Plakoglobin (also known as? -catenin) is a member of the Armadillo family of proteins and a paralog of β -catenin. Plakoglobin is a component of both the adherens junctions and desmosomes, and therefore plays a vital role in the regulation of cell-cell adhesion. Similar to β -catenin, plakoglobin is capable of participating in cell signaling in addition to its role in cell-cell adhesion. In this context, β -catenin has a well-documented oncogenic potential as a component of the Wnt signaling pathway. In contrast, while some studies have suggested a tumor promoting activity of plakoglobin in a cell/malignancy specific context, it generally acts as a tumor/metastasis suppressor. How plakoglobin acts as a growth/metastasis inhibitory protein has remained, until recently, unclear. Recent evidence suggests that plakoglobin may suppress tumorigenesis and metastasis by multiple mechanisms, including the suppression of oncogenic signaling, interactions with various proteins involved in tumorigenesis and metastasis, and the regulation of the expression of genes involved in these processes. This review is primarily focused on various mechanisms by which plakoglobin may inhibit tumorigenesis and metastasis.

Plakophilin3 increases desmosome assembly, size and stability by increasing expression of desmocollin2

Previous reports show that the desmosomal plaque protein plakophilin3 (PKP3) is essential for desmosome formation. Here, we report that PKP3 over-expression decreases calcium dependency for de novo desmosome formation and makes existing cell-cell adhesion junctions more resilient in low calcium medium due to an increase in desmocollin2 expression. PKP3 overexpression increases the stability of other desmosomal proteins independently of the increase in DSC2 levels and regulates desmosome formation and stability by a multimodal mechanism affecting transcription, protein stability and cell border localization of desmosomal proteins.

Intermediate Filaments and the Plasma Membrane

A variety of intermediate filament (IF) types show intricate association with plasma membrane proteins, including receptors and adhesion molecules. The molecular basis of linkage of IFs to desmosomes at sites of cell-cell interaction and hemidesmosomes at sites of cell-matrix adhesion has been elucidated and involves IF-associated proteins. However, IFs also interact with focal adhesions and cell-surface molecules, including dystroglycan. Through such membrane interactions, it is well accepted that IFs play important roles in the establishment and maintenance of tissue integrity. However, by organizing cell-surface complexes, IFs likely regulate, albeit indirectly, signaling pathways that are key to tissue homeostasis and repair.

Pathophysiology of the Desmo-Adhesome

Advances in our understanding of desmosomal diseases have provided a clear demonstration of the key role played by desmosomes in tissue and organ physiology, highlighting the importance of their dynamic and finely regulated structure. In this context, non-desmosomal regulatory molecules have acquired increasing relevance in the study of this organelle resulting in extending the desmosomal interactome, named the "desmo-adhesome." Spatiotemporal changes in the expression and regulation of the desmo-adhesome underlie a number of genetic, infectious, autoimmune, and malignant conditions. The aim of the present article was to examine the structural and functional relationship of the desmosome, by providing a comprehensive, yet focused overview of the constituents targeted in human disease. The inclusion of the novel regulatory network in the desmo-adhesome pathophysiology opens new avenues to a deeper understanding of desmosomal diseases, potentially unveiling pathogenic mechanisms waiting to be explored. J. Cell. Physiol. 232: 496-505, 2017. © 2016 Wiley Periodicals, Inc.

Antagonistic Regulation of Intercellular Cohesion by Plakophilins 1 and 3

Desmosomes are cell-cell adhesive structures essential for tissue integrity of the epidermis and the heart. Their constituents belong to multigene families giving rise to desmosomes of variable composition. So far, the functional significance of context-dependent composition in desmosome formation, dynamics, or stability during epidermal differentiation is incompletely understood. In this comparative study, we have uncovered unique and partially antagonistic functions of plakophilins 1 and 3 that are both expressed in the murine epidermis. These plakophilins differ in their localization patterns and kinetics during de novo desmosome formation and are regulated by distinct mechanisms. Moreover, plakophilin 3-containing desmosomes are more dynamic than desmosomes that contain predominantly plakophilin 1. Further, we show that Ca(2+)-independence of desmosomes strictly depends on plakophilin 1, whereas elevated levels of plakophilin 3 prevent the formation of hyperadhesive desmosomes in a protein kinase C alpha-dependent manner, even in the presence of plakophilin 1. Our study demonstrates that the balance between plakophilins 1 and 3 determines the context-dependent properties of epidermal desmosomes. In this setting, plakophilin 1 provides stable intercellular cohesion that resists mechanical stress, whereas plakophilin 3 confers dynamics as required during tissue homeostasis and repair. Our data have implications for the role of plakophilins in carcinogenesis.

MMP7 is required to mediate cell invasion and tumor formation upon Plakophilin3 loss

Plakophilin3 (PKP3) loss results in increased transformation in multiple cell lines in vitro and increased tumor formation in vivo. A microarray analysis performed in the PKP3 knockdown clones, identified an inflammation associated gene signature in cell lines derived from stratified epithelia as opposed to cell lines derived from simple epithelia. However, in contrast to the inflammation associated gene signature, the expression of MMP7 was increased upon PKP3 knockdown in all the cell lines tested. Using vector driven RNA interference, it was demonstrated that MMP7 was required for in-vitro cell migration and invasion and tumor formation in vivo. The increase in MMP7 levels was due to the increase in levels of the Phosphatase of Regenerating Liver3 (PRL3), which is observed upon PKP3 loss. The results suggest that MMP7 over-expression may be one of the mechanisms by which PKP3 loss leads to increased cell invasion and tumor formation.

Dsp rul: a spontaneous mouse mutation in desmoplakin as a model of Carvajal-Huerta syndrome

Studies of spontaneous mutations in mice have provided valuable disease models and important insights into the mechanisms of human disease. Ruffled (rul) is a new autosomal recessive mutation causing abnormal hair coat in mice. The rul allele arose spontaneously in the RB156Bnr/EiJ inbred mouse strain. In addition to an abnormal coat texture, we found diffuse epidermal blistering, abnormal electrocardiograms (ECGs), and ventricular fibrosis in mutant animals. Using high-throughput sequencing (HTS) we found a frameshift mutation at 38,288,978bp of chromosome 13 in the desmoplakin gene (Dsp). The predicted mutant protein is truncated at the c-terminus and missing the majority of the plakin repeat domain. The phenotypes found in Dsp(rul) mice closely model a rare human disorder, Carvajal-Huerta syndrome. Carvajal-Huerta syndrome (CHS) is a rare cardiocutaneous disorder that presents in humans with wooly hair, palmoplantar keratoderma and ventricular cardiomyopathy. CHS results from an autosomal recessive mutation on the 3' end of desmoplakin (DSP) truncating the full length protein. The Dsp(rul) mouse provides a new model to investigate the pathogenesis of CHS, as well as the underlying basic biology of the adhesion molecules coded by the desmosomal genes.

Plakophilins 1 and 3 bind to FXR1 and thereby influence the mRNA stability of desmosomal proteins

Plakophilins 1 and 3 (PKP1/3) are members of the arm repeat family of catenin proteins and serve as structural components of desmosomes, which are important for cell-cell-adhesion. In addition, PKP1/3 occur as soluble proteins outside desmosomes, yet their role in the cytoplasm is not known. We found that cytoplasmic PKP1/3 coprecipitated with the RNA-binding proteins FXR1, G3BP, PABPC1, and UPF1, and these PKP1/3 complexes also comprised desmoplakin and PKP2 mRNAs. Moreover, we showed that the interaction of PKP1/3 with G3BP, PABPC1, and UPF1 but not with FXR1 was RNase sensitive. To address the cytoplasmic function of PKP1/3, we performed gain-and-loss-of-function studies. Both PKP1 and PKP3 knockdown cell lines showed reduced protein and mRNA levels for desmoplakin and PKP2. Whereas global rates of translation were unaffected, desmoplakin and PKP2 mRNA were destabilized. Furthermore, binding of PKP1/3 to FXR1 was RNA independent, and both PKP3 and FXR1 stabilized PKP2 mRNA. Our results demonstrate that cytoplasmic PKP1/3 are components of mRNA ribonucleoprotein particles and act as posttranscriptional regulators of gene expression.

Potential prognostic, diagnostic and therapeutic markers for human gastric cancer

The high incidence of gastric cancer (GC) and its consequent mortality rate severely threaten human health. GC is frequently not diagnosed until a relatively advanced stage. Surgery is the only potentially curative treatment. Thus, early screening and diagnosis are critical for improving prognoses in patients with GC. Gastroscopy with biopsy is an appropriate method capable of aiding the diagnosis of specific early GC tumor types; however, the stress caused by this method together with it being excessively expensive makes it difficult to use it as a routine method for screening for GC on a population basis. The currently used tumor marker assays for detecting GC are simple and rapid, but their use is limited by their low sensitivity and specificity. In recent years, several markers have been identified and tested for their clinical relevance in the management of GC. Here, we review the serum-based tumor markers for GC and their clinical significance, focusing on discoveries from microarray/proteomics research. We also review tissue-based GC tumor markers and their clinical application, focusing on discoveries from immunohistochemical research. This review provides a brief description of various tumor markers for the purposes of diagnosis, prognosis and therapeutics, and we include markers already in clinical practice and various forthcoming biomarkers.

Plakophilin 3 mediates Rap1-dependent desmosome assembly and adherens junction maturation

Desmosomal Armadillo family member Pkp3 is established as a coordinator of desmosome and adherens junction assembly and maturation through its physical and functional association with Rap1. It thus functions in a manner distinct from the closely related Pkp2.

The pathways driving desmosome and adherens junction assembly are temporally and spatially coordinated, but how they are functionally coupled is poorly understood. Here we show that the Armadillo protein plakophilin 3 (Pkp3) mediates both desmosome assembly and E-cadherin maturation through Rap1 GTPase, thus functioning in a manner distinct from the closely related plakophilin 2 (Pkp2). Whereas Pkp2 and Pkp3 share the ability to mediate the initial phase of desmoplakin (DP) accumulation at sites of cell–cell contact, they play distinct roles in later steps: Pkp3 is required for assembly of a cytoplasmic population of DP-enriched junction precursors, whereas Pkp2 is required for transfer of the precursors to the membrane. Moreover, Pkp3 forms a complex with Rap1 GTPase, promoting its activation and facilitating desmosome assembly. We show further that Pkp3 deficiency causes disruption of an E-cadherin/Rap1 complex required for adherens junction sealing. These findings reveal Pkp3 as a coordinator of desmosome and adherens junction assembly and maturation through its functional association with Rap1.

In VivoFunction of Desmosomes

Desmosomes are morphologically and biochemically defined cell-cell junctions that are required for maintaining the mechanical integrity of skin and the heart in adult mammals. Furthermore, since mice with null mutations in desmosomal plaque proteins (plakoglobin and desmoplakin) die in utero, it is also evident that desmosomes are indispensable for normal embryonic development. This review focuses on the role of desmosomes in vivo. We will summarize the effects of mutations in desmosomal genes on pre- and post-embryonic development of mouse and man and discuss recent findings relating to the specific role of desmosomal cadherins in skin differentiation and homeostasis.

Palmitoylation of plakophilin is required for desmosome assembly

Desmosomes are prominent adhesive junctions found in various epithelial tissues. The cytoplasmic domains of desmosomal cadherins interact with a host of desmosomal plaque proteins, including plakophilins, plakoglobin and desmoplakin, which, in turn, recruit the intermediate filament cytoskeleton to sites of cell-cell contact. Although the individual components of the desmosome are known, mechanisms regulating the assembly of this junction are poorly understood. Protein palmitoylation is a posttranslational lipid modification that plays an important role in protein trafficking and function. Here, we demonstrate that multiple desmosomal components are palmitoylated in vivo. Pharmacologic inhibition of palmitoylation disrupts desmosome assembly at cell-cell borders. We mapped the site of plakophilin palmitoylation to a conserved cysteine residue present in the armadillo repeat domain. Mutation of this single cysteine residue prevents palmitoylation, disrupts plakophilin incorporation into the desmosomal plaque and prevents plakophilin-dependent desmosome assembly. Finally, plakophilin mutants unable to become palmitoylated act in a dominant-negative manner to disrupt proper localization of endogenous desmosome components and decrease desmosomal adhesion. Taken together, these data demonstrate that palmitoylation of desmosomal components is important for desmosome assembly and adhesion.

Coherent somatic mutation in autoimmune disease

BACKGROUND

Many aspects of autoimmune disease are not well understood, including the specificities of autoimmune targets, and patterns of co-morbidity and cross-heritability across diseases. Prior work has provided evidence that somatic mutation caused by gene conversion and deletion at segmentally duplicated loci is relevant to several diseases. Simple tandem repeat (STR) sequence is highly mutable, both somatically and in the germ-line, and somatic STR mutations are observed under inflammation.

RESULTS

Protein-coding genes spanning STRs having markers of mutability, including germ-line variability, high total length, repeat count and/or repeat similarity, are evaluated in the context of autoimmunity. For the initiation of autoimmune disease, antigens whose autoantibodies are the first observed in a disease, termed primary autoantigens, are informative. Three primary autoantigens, thyroid peroxidase (TPO), phogrin (PTPRN2) and filaggrin (FLG), include STRs that are among the eleven longest STRs spanned by protein-coding genes. This association of primary autoantigens with long STR sequence is highly significant (p<3.0x10(-7)). Long STRs occur within twenty genes that are associated with sixteen common autoimmune diseases and atherosclerosis. The repeat within the TTC34 gene is an outlier in terms of length and a link with systemic lupus erythematosus is proposed.

CONCLUSIONS

The results support the hypothesis that many autoimmune diseases are triggered by immune responses to proteins whose DNA sequence mutates somatically in a coherent, consistent fashion. Other autoimmune diseases may be caused by coherent somatic mutations in immune cells. The coherent somatic mutation hypothesis has the potential to be a comprehensive explanation for the initiation of many autoimmune diseases.

14-3-3γ-Mediated transport of plakoglobin to the cell border is required for the initiation of desmosome assembly in vitro and in vivo

The regulation of cell-cell adhesion is important for the processes of tissue formation and morphogenesis. Here, we report that loss of 14-3-3γ leads to a decrease in cell-cell adhesion and a defect in the transport of plakoglobin and other desmosomal proteins to the cell border in HCT116 cells and cells of the mouse testis. 14-3-3γ binds to plakoglobin in a PKCμ-dependent fashion, resulting in microtubule-dependent transport of plakoglobin to cell borders. Transport of plakoglobin to the border is dependent on the KIF5B-KLC1 complex. Knockdown of KIF5B in HCT116 cells, or in the mouse testis, results in a phenotype similar to that observed upon 14-3-3γ knockdown. Our results suggest that loss of 14-3-3γ leads to decreased desmosome formation and a decrease in cell-cell adhesion in vitro, and in the mouse testis in vivo, leading to defects in testis organization and spermatogenesis.